An electroluminescent device (EL device) is a self-light-emitting display device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak in 1987, by using small aromatic diamine molecules and aluminum complexes as materials for forming a light-emitting layer (see Appl. Phys. Lett. 51, 913, 1987).
An organic EL device (OLED) changes electric energy into light by applying electricity to an organic electroluminescent material, and commonly comprises an anode, a cathode, and an organic layer formed between the two electrodes. The organic layer of the organic EL device may comprise a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer (containing host and dopant materials), an electron buffer layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc. The materials used in the organic layer can be classified into a hole injection material, a hole transport material, an electron blocking material, a light-emitting material, an electron buffer material, a hole blocking material, an electron transport material, an electron injection material, etc., depending on their functions. In the organic EL device, holes from the anode and electrons from the cathode are injected into a light-emitting layer by the application of electric voltage, and excitons having high energy are produced by the recombination of the holes and electrons. The organic light-emitting compound moves into an excited state by the energy and emits light from an energy when the organic light-emitting compound returns to the ground state from the excited state.
In an organic electroluminescent device, research is continuing to improve the performance of an organic electroluminescent device by using a material suitable as a material used for each layer.
An electron transport material actively transports electrons from a cathode to a light-emitting layer and inhibits transport of holes which are not recombined in the light-emitting layer to increase recombination opportunity of holes and electrons in the light-emitting layer. Thus, electron-affinitive materials are used as an electron transport material. Organic metal complexes having light-emitting function such as Alq3 are excellent in transporting electrons, and thus have been conventionally used as an electron transport material. However, Alq3 has problems in that it moves to other layers and shows reduction of color purity when used in blue light-emitting devices. Therefore, new electron transport materials have been required, which do not have the above problems, are highly electron-affinitive, and quickly transport electrons in organic EL devices to provide organic EL devices having high luminous efficiency.
Further, the electron buffer layer is a layer for solving the problem of a change in luminance caused by the change of a current characteristic of the device when exposed to a high temperature during a process of producing a panel. In order to obtain a similar current characteristic and a stability against high temperature exposure compared to a device without an electron buffering layer, the characteristic of the compound comprised in the electron buffer layer is important.
Meanwhile, an organic electroluminescent material can form layers by deposition, the deposition process commonly being performed at a high temperature above 130° C. Thus, an organic electroluminescent compound capable of withstanding heat at high temperatures is required.
Japanese Patent Laid-Open No. 2014-160813 A discloses a compound in which a pyrrole, a plurality of benzene rings, and a 7-membered ring are fused, as a compound for a host material of an organic electroluminescent device. However, the aforementioned document does not specifically disclose a compound in which an imidazole, a plurality of benzene rings, and a 7-membered ring are fused. Furthermore, it does not disclose specific examples using such a compound as electron buffer materials and electron transport materials.